gi6 'Appendix II. Rider Engine, Fig. 879, which has been much used for small powers, c is a displacer, and D the power plunger, their cranks being about 90° apart. At F is a fire (hot body), at E a water jacket (refrigerator), and H is the regenerator; the cycle being essentially that of Stirling, and the operations as follows :— 1. Air compressed in D takes heat and expands: thus work is done by the gas. 2. The air is transferred to c nearly at constant volume, giving heat to H. 3. Air under c is compressed, losing heat to E : thus work is- done on the gas. 4. The air is re-transferred to D at nearly constant volume,. taking heat from H.